This application claims priority from Japanese Patent Application No. 2000-243658, filed on Aug. 11, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a method of manufacturing a molded product having a fine structure from a micro-phase separated structure comprising a block copolymer or a graft copolymer, and molded products prepared thereby.
2. Discussion of the Background
Known methods of preparing a molded product having a fine structure on the nanometer scale include utilizing the micro-phase separated of a block copolymer or a graft copolymer (see, for example, Vanessa Z.-H. Chan, et al, Science, 286, 1716 (1999)). In this process, a single specific phase of a micro-phase separated structure may be selectively removed to provide a molded product having a fibrous structure, a thin film structure or a porous structure. In addition, a phase of a micro-phase separated membrane formed on a substrate may be selectively removed to provide a porous membrane or dots arranged in an array, and the porous membrane or the dot array may then be used as an etching mask to prepare a finely fabricated surface on a substrate.
Since each the polymer chains of the block copolymer or the graft copolymer are chemically bonded to each other, in order to selectively remove a specified phase of the micro-phase separated structure, the main chain of the polymer phase to be removed must be decomposed selectively. The decomposed polymer chain may be removed by various processes, such as vaporization upon heating or leaching (i.e., removal by dissolution or suspension of the decomposed polymer chain) with an organic solvent or water. However, removing the decomposed polymer chain by heat treatment or by leaching with organic solvents or water tends to cause problems in forming the desired fine structure. For example, if the heat treatment is conducted at the typical high temperature range of 200xc2x0 C. or higher, in order to thermally decompose and vaporize the desired polymer phase, the fine structure of the remaining polymer phase, after heating, may be thermally degraded, for example by oxidation, or by thermal deformation if the heat treatment temperature is higher than the glass transition temperature of the remaining polymer phase. Accordingly, the molded products that can be produced by a process including a high temperature heat treatment step, as described above, are limited to molded products comprising a heat resistant polymers having high glass transition temperature, such as polyimides. Thus, such heat treatment processes may not be generally applicable to other materials.
The leaching process has the advantage of removing the decomposed polymer chains at a relatively low temperature. However, leaching is also relatively slow and often requires a long time, for example one day or more, and the throughput is low. In particular, it is extremely difficult to leach decomposed polymer from the inside of a molded product having a thickness, for example, of several millimeters or more. Furthermore, leaching processes require large amounts of organic solvents and therefore produce large amounts of liquid wastes after leaching. Thus leaching processes typically also pose environmental problems. In addition, residual organic solvent or water used in the leaching process tends to remain in the fine structure of the molded product. That is, fragments of the decomposed polymer chain remain on the inner wall of the fine structure, i.e., the fine pores formed by the removal of the specified phase. Moreover, it is difficult to completely remove these residues of polymer chain fragments and solvent (e.g., water or organic solvent) completely, because the inner wall of the microstructure and the residual polymer fragments have an extremely high affinity for the solvent.
Such residual solvents may cause various problems when such molded products are used, and therefore solvents are typically removed by thoroughly drying the molded product. However, since the fine structure of the molded product tends to be swollen and soften by the residual solvent, the fine structure may collapse if the remaining polymer forming the fine structure is heated to a temperature above its glass transition temperature.
As discussed above, producing a molded product having a fine structure by leaching and removing a decomposed polymer chain from a micro-phase separated structure with a solvent has the advantage that the decomposed polymer may be removed at a relatively low temperature. However, leaching processes have the disadvantage of requiring a long time, requiring large amounts of solvent, and producing large amounts of liquid waste. In addition, since the inner wall of the fine structure, i.e. the voids, inherently have an extremely high affinity to the leaching solvent in the fine structure, the solvent tends to remain therein, and the fine structure tends to collapse, since the inner wall of the fine structure becomes swollen and softened. Further, since the fine structure of the molded product comprises a polymer, it is prone to thermal deformation, so that sufficient heat cannot be applied to dry the residual solvent.
Accordingly, it is an object of the present invention to provide a method of producing a molded product having a fine structure comprising preparing a structure having two or more phases comprising a block copolymer or a graft copolymer, decomposing the polymer chains of at least one phase of the structure, and cleaning the structure with a supercritical fluid or a sub-critical fluid, thereby removing the decomposed polymer chains. The cleaning step may be applied at a relatively low temperatures and over a short period of time, and provides a molded porous product with a fine structure having relatively low levels of residual solvent, without using large amounts of organic solvents, and forming only small amounts of liquid wastes.
It is another object of the present invention to provide molded products having a fine porous structure, using the method of the present invention, such as filters, battery separators, contact lenses, cladding layers of optical fibers, etc.